Regulation of Nitrogen Utilization and Lodging Resistance of Rice in Northeast China Through Continuous Straw Return and Nitrogen Fertilizer Application

Abstract

Combining straw return with nitrogen fertilizer application is an effective strategy to enhance farmland productivity, improve soil structure, and mitigate climate change. Although straw return practices are widely recommended in agricultural ecosystems targeting sustainable agriculture, few studies have investigated the combined effects of consecutive years of straw return and nitrogen-fertilizer interactions on rice yield, nitrogen use, and lodging resistance, as well as the potential interactions among these variables. To investigate the effects of consecutive years of rice straw return and nitrogen fertilizer inputs on rice growth, a straw return experiment was conducted in 2021–2022 in Northeast China. The test crop was rice (cv. Jinongda No. 667), with four nitrogen fertilizer levels: 0 kg/ha (N0), 125 kg/ha (N1), 150 kg/ha (N2), and 175 kg/ha (N3). Five straw-return treatments were applied: no straw (S0), straw return to the field for one year (S1), continuous straw return to the field for two years (S2), continuous straw return to the field for three years (S3), and continuous straw return to the field for four years (S4). Results indicated that under the same straw return year, the N3 yield, nitrogen accumulation, nitrogen use efficiency, and apparent utilization were the highest. Under the same nitrogen treatment condition, S1 significantly reduced yield, nitrogen accumulation, nitrogen use efficiency, apparent nitrogen utilization, and lodging index compared to S0. However, under N3 conditions, S3 did not significantly differ from S0. Both S3 and N3 enhanced nitrogen uptake, translocation, and accumulation in rice. Their significant interactive effect increased yield while simultaneously enhancing the lodging resistance and stem strength. The study findings highlighted the effects of years of straw return and nitrogen fertilizer application on crop yield and resistance traits. They further demonstrated that the combination of straw return and optimized nitrogen fertilizer inputs could improve resource utilization and result in a high-yielding and efficient crop population.

1. Introduction

Crop straw serves as the primary raw material for nutrient recycling and is a fundamental component of conservation tillage systems. The efficient utilization of straw is a viable strategy for promoting organic and sustainable agricultural practices. Numerous studies have confirmed that straw incorporation can significantly enhance soil nutrient use efficiency, improve soil aggregate stability, facilitate nutrient assimilation, and ultimately boost crop productivity [1,2,3,4]. However, microorganisms involved in straw decomposition consume mineral nitrogen (N) that would otherwise be available to cereal crops, leading to competition for N during early crop growth. This adversely affects early crop development [5].

Successive years of straw return can substantially increase soil organic carbon content, and the addition of fresh straw to the soil may further enhance primary organic carbon mineralization by stimulating microbial activity through nutrient tapping. Previous studies have shown that returning crop straw to the field for three consecutive years can increase the soil organic matter content [6]. Although straw return to the field enhances soil texture, it does not meet the crop nutrient demands. Thus, under straw return to the field conditions, nitrogen application can significantly increase maize yield and nitrogen uptake [7,8]. Compared with no straw return to the field, consecutive years of straw return to the field can increase wheat and corn yields [9]. Moreover, combining consecutive years of straw return to the field with nitrogen fertilizer application effectively improves nitrogen use efficiency (NUE) and reduces nitrogen loss. Therefore, combining straw recycling with nitrogen fertilizer application is a common practice that both preserves soil fertility and enhances crop yields [10].

Nitrogen uptake, dry matter accumulation, and nutrient transport in plants are critical to crop yield [11]. Adequate leaf N increases dry matter accumulation and crop yield [12,13]. Bonelli found that applying N fertilizer improved leaf area index, chlorophyll content, and leaf N content [14]. Moderate nitrogen application and straw return to the field can effectively regulate the canopy structure and increase the amount of photosynthesis [15,16]. Simultaneously, synchronizing nitrogen supply with soil moisture and implementing appropriate straw incorporation techniques can further enhance crop productivity [17]. Nitrogen is one of the key factors influencing rice yield. Enhancing nitrogen-use efficiency can boost yields by 30%~50% [18]. Making it crucial to improve the utilization rate of nitrogen fertilizers, and to reasonably transport nitrogen fertilizers to improve rice yield [19].

Nitrogen application also influences stunting resistance by affecting plant height and lignin and cellulose content in stem sheaths [20,21]. Excess nitrogen increases plant height and tiller number, thereby elevating lodging risk [22,23]. Similar effects of nitrogen application on the yield of aromatic rice have recently been reported [24,25], but limited research has been conducted on the resistance of aromatic rice to buckling under different nitrogen application levels. Proper fertilizer transport can shorten the length of rice basal internodes, increase internode and cell wall thickness, lower the plant’s center of gravity [26], and improve stalk bending resistance. This increases the mechanical strength of the rice stalks and reduces the lodging index, thus increasing the yield.

Previous studies have demonstrated that tillage practices significantly influence rice growth, yield, and lodging resistance. Straw return combined with low nitrogen input promotes root distribution in deep soil, thereby improving nitrogen use efficiency in winter wheat [27]. Long-term straw return to the field produces higher crop yields and nitrogen uptake, and increases soil productivity year after year [28]. Long-term straw fertilization increases nutrient and organic carbon inputs, has great potential to improve soil fertility, and boosts microbial population abundance [5]. To date, most studies have focused on the effects of straw addition and nitrogen fertilization on the soil microcosm environment, on-farm carbon sequestration, and emission reduction. However, fewer studies have examined the effects of different straw return years and nitrogen application on rice light yield, resource use efficiency, and lodging resistance [29,30]. Therefore, this study conducted a two-year field experiment with the following main objectives: (1) To investigate the effects of straw return and nitrogen fertilizer application rate on rice yield; (2) to analyze the effects of straw return and nitrogen fertilizer application rate on rice nitrogen use efficiency; (3) to clarify the effects of continuous straw return for many years on rice lodging resistance. This study aims to provide a theoretical basis for rational straw return practices and sustainable development of farmland in Northeast China.

2. Materials and Methods

2.1. Experimental Site and Experimental Materials

The trials were conducted in 2021 and 2022 at the National Crop Variety Validation Characterization Station of Jilin Agricultural University, Changchun City, Jilin Province (125°24′ E, 43°48′ N). The site is located in the Northeast Plain, which has a temperate monsoon climate. The basic soil nutrients in the 0–20 cm layer are: organic matter content 9.6 g/kg, alkaline hydrolyzable nitrogen content 27.45 mg/kg, readily available potassium content 145.23 mg/kg, readily available phosphorus content 18.54 mg/kg, and pH value of 6.70. Annual rainfall was 860 mm in 2021 and 600 mm in 2022.

2.2. Experimental Design

Rice (cv. Jinongda No. 667, China Rice Data Center, No. 20243224, https://www.ricedata.cn/variety/varis/619880.htm, accessed on 1 March 2021) was selected as the rice variety for testing. The rice variety Jinongda 667 is suitable for cultivation in the central region of Jilin Province. A two-year randomized block experiment with three replications was conducted. This study employed a flooding irrigation regime. Three treatments were applied in the 2021 experiment: S0 (no straw), S1 straw returned in 2021 (straw return to the field for one year), S3 straw returned in 2021, 2020, and 2019 (continuous straw return to the field for three years). Each treatment included four nitrogen fertilizer levels, namely, 0 kg/ha (N0), 125 kg/ha (N1), 150 kg/ha (N2), and 175 kg/ha (N3). A total of five treatments were applied in the 2022 trial, including S0 (no straw), S1 straw returned in 2021 (straw returned to the field for one year), S2 straw returned in 2021 and 2020 (continuous straw return to the field for two years), S3 straw returned in 2021, 2020, and 2019 (continuous straw return to the field for three years), S4 straw returned in 2021, 2020, 2019, and 2018 (continuous straw return to the field for four years). Nitrogen fertilizer was applied at 175 kg/ha in 2022. 75 kg/ha each of P₂O₅ (calcium superphosphate) and K₂O (potassium chloride) were applied at once as base fertilizer for each treatment in 2021 and 2022. Nitrogen fertilizer (urea) was applied in three splits as basal fertilizer:tiller fertilizer:ear fertilizer = 6:3:1. The straw used in the experiment was rice straw harvested from the previous year, and the straw incorporation rate was set at 4500 kg/ha (dry weight basis). After air-drying under natural conditions, the straw was cut into small sections of 5–7 cm in length using a straw grinder. The crushed straw was evenly spread and incorporated into the 0–20 cm soil layer using a rotary tiller (Tianma Tillage Machinery Co., Ltd., Luoyang, Henan, China)during the experimental season. In 2021, seedlings were planted on 12 April, transplanted on 24 May when plants were at the 3-leaves and 1-heart stage, and harvested on 29 September. In 2022, seedlings were planted on 11 April, transplanted on 23 May at the 3-leaves and 1-heart stage, and harvested on 28 September. In both years, seedlings were transplanted manually at a spacing of 30 cm × 13 cm, with 5 plants per hole. Individual irrigation and drainage were available between each experimental plot, and other management was carried out according to local conventional high-yield cultivation methods.

2.3. Measurement Index and Methods

2.3.1. Measurement of Tiller Dynamics

Ten representative plants from each treatment were selected as sample plants. The number of tillers of the sample plants was examined at 7 d intervals starting from the tillering stage until the end of the examination, when the number of tillers decreased twice consecutively.

2.3.2. Measurement of Plant Height

Ten representative plants with uniform growth were selected from each treatment as sample plants. The plant height of the samples was examined at 7 d intervals starting from the tillering stage. The height from the soil surface to the tip of the tallest leaf of each plant was measured before spiking. After spiking, height was measured from the soil surface to the tip of the tallest spike (excluding the awn).

2.3.3. Determination of Dry Matter Weight

Three rice plants were sampled from each treatment at the tillering, nodulation, full heading, twenty days after full heading, and maturity, respectively. The roots were rinsed clean with running water, and the plants were separated according to stems, leaves, and spikes. The samples were then killed in an oven at 105 °C for 30 min, and then dried at 80 °C to a constant weight, and the dry weights of the retained samples were weighed after cooling at room temperature. The following parameters were then calculated. TVDM: transport volume of dry matter; TRDM: transport rate of dry matter; spike IDM: increase in dry matter; CRDM: contribution rate of dry matter transport to the panicle; TDMA: total dry matter accumulation. (HS: heading stage; MS: maturity stage).

The formulas are as follows:

$$\begin{array}{cc}TVDM(\mathrm{k}\mathrm{g}/\mathrm{h}\mathrm{a}) =& \left(HS\right)Dry matter accumulation(\mathrm{k}\mathrm{g}/\mathrm{h}\mathrm{a})\hfill \\ & - \left(MS\right)Dry matter accumulation(\mathrm{k}\mathrm{g}/\mathrm{h}\mathrm{a})\hfill \end{array}$$

(1)

$$\begin{array}{cc}\hfill TRDM(\%)=& TVDM(\mathrm{k}\mathrm{g}/\mathrm{h}\mathrm{a})\hfill \\ & ÷\left(HS\right)Dry matter accumulation(\mathrm{k}\mathrm{g}/\mathrm{h}\mathrm{a})\times 100\%\end{array}$$

(2)

$$\begin{array}{cc}\hfill Spike IDM(\mathrm{k}\mathrm{g}/\mathrm{h}\mathrm{a})& \\ \hfill =& \left(MS\right)Dry matter accumulation of spike(\mathrm{k}\mathrm{g}/\mathrm{h}\mathrm{a})\hfill \\ \hfill -& \left(HS\right)Dry matter accumulation of spike(\mathrm{k}\mathrm{g}/\mathrm{h}\mathrm{a})\hfill \end{array}$$

(3)

$$\begin{array}{cc}\hfill CRDM(\%)=& \left(\left(Stem\right)TVDM\right(\mathrm{k}\mathrm{g}/\mathrm{h}\mathrm{a})\hfill \\ & +\left(Leaf\right)TVDM(\mathrm{k}\mathrm{g}/\mathrm{h}\mathrm{a}))/Spike IDM(\mathrm{k}\mathrm{g}/\mathrm{h}\mathrm{a})\times 100\%\hfill \end{array}$$

(4)

$$\begin{array}{cc}\hfill TDMA(\mathrm{k}\mathrm{g}/\mathrm{h}\mathrm{a})=& \left(Stem\right)Dry matter accumulation(\mathrm{k}\mathrm{g}/\mathrm{h}\mathrm{a})\hfill \\ & +\left(Leaf\right)Dry matter accumulation(\mathrm{k}\mathrm{g}/\mathrm{h}\mathrm{a})\hfill \\ & +\left(Spike\right)Dry matter accumulation (\mathrm{k}\mathrm{g}/\mathrm{h}\mathrm{a})\hfill \end{array}$$

(5)

2.3.4. Determination of Nitrogen Content

Samples treated in Section 2.3.3 were dried, crushed using a sample mill, and passed through a 0.50 mm sieve. The crushed sample was weighed at 0.50 g at the bottom of the digestion tube using a 1 in 10,000 balance, and 5 mL of concentrated sulfuric acid was added, then the mixture was left to stand overnight. The nitrogen content in plant organs was determined using an automated Kjeldahl nitrogen analyzer (FOSS-8400, the Foss Group Corporation of Denmark, Hilloerod, Denmark ) following digestion with the H₂SO₄-H₂O₂ method. The following parameters were then calculated. NTA: nitrogen translocation amount. NTE: nitrogen translocation efficiency. NTCR: nitrogen contribution rate of translocation; NHI: nitrogen harvest index; NBPE: nitrogen biomass production efficiency; NPP: nitrogen partial productivity; NAE: nitrogen agronomic efficiency; NRE: nitrogen recovery efficiency; NAUE: nitrogen apparent utilization efficiency; NPE: nitrogen physiological efficiency.

The formulas are as follows:

$$\begin{array}{cc}NTA(\mathrm{k}\mathrm{g}/\mathrm{h}\mathrm{a}) =& \left(HS\right)(Nitrogen content in dry matter(\%)\hfill \\ & \times Dry matter accumulation(\mathrm{k}\mathrm{g}/\mathrm{h}\mathrm{a}))\hfill \\ & -\left(MS\right)(Nitrogen content in dry matter(\%)\hfill \\ & \times Dry matter accumulation(\mathrm{k}\mathrm{g}/\mathrm{h}\mathrm{a}))\hfill \end{array}$$

(6)

$$\begin{array}{cc}NTE(\%)=& NTA ÷\left(HS\right)(Nitrogen content in dry matter(\%)\hfill \\ & \times Dry matter accumulation(\mathrm{k}\mathrm{g}/\mathrm{h}\mathrm{a}))\times 100\%\hfill \end{array}$$

(7)

$$\begin{array}{cc}NTCR(\%)=& NTA/\left(MS\right)\left(\right(Spike)Nitrogen content in dry matter(\%)\hfill \\ & \times \left(Spike\right)Dry matter accumulation(\mathrm{k}\mathrm{g}/\mathrm{h}\mathrm{a}))\times 100\%\hfill \end{array}$$

(8)

$$\begin{array}{cc}NHI(\%)=& \left(\right(Spike)Nitrogen content in dry matter(\%)\hfill \\ & /Total nitrogen uptake of the plant(\mathrm{k}\mathrm{g}/\mathrm{h}\mathrm{a}))\times 100\%\hfill \end{array}$$

(9)

$$\begin{array}{cc}\left(MS\right)NBPE(\mathrm{k}\mathrm{g}/\mathrm{k}\mathrm{g})& \\ \hfill =& \left(Plant\right)Dry matter accumulation\hfill \\ \hfill ÷& \left(Plant\right)Nitrogen content in dry matter\hfill \end{array}$$

(10)

$$NPP(\mathrm{k}\mathrm{g}/\mathrm{k}\mathrm{g})=Grain yield÷Nitrogen application rate$$

(11)

$$\begin{array}{cc}NAE(\%)=& (Grain yield in the nitrogen application areas\hfill \\ & -Grain yield in nitrogen free areas)\hfill \\ & ÷Nitrogen application rate\hfill \end{array}$$

(12)

$$\begin{array}{c}NRE(\%)\hfill \\ =(Nitrogen content in dry matter in the nitrogen application areas\hfill \\ - Nitrogen content in dry matter in nitrogen free areas)\hfill \\ ÷ Nitrogen application rate\times 100\%\hfill \end{array}$$

(13)

$$\begin{array}{c}NAUE(\%)\hfill \\ =(Total nitrogen uptake of the plant in the nitrogen application areas(\mathrm{k}\mathrm{g}/\mathrm{h}\mathrm{a})\hfill \\ -Total nitrogen uptake of the plant in nitrogen free areas(\mathrm{k}\mathrm{g}/\mathrm{h}\mathrm{a}))\hfill \\ ÷ Nitrogen application rate\times 100\%\hfill \end{array}$$

(14)

$$\begin{array}{c}NPE(\%)\hfill \\ =\left(\right(Grain yield in the nitrogen application areas\hfill \\ - Grain yield in nitrogen free areas)\hfill \\ /(Nitrogen content in dry matter in the nitrogen application areas\hfill \\ - Nitrogen content in dry matter in nitrogen free areas\left)\right)\times 100\%\hfill \end{array}$$

(15)

2.3.5. Indicators Related to Stalk Morphology and Mechanical Traits Were Measured

The direction of this research is stem lodging. Measurements recorded included, center of gravity height, plant height, the length and fresh weight from the second basal internode to the panicle apex, internode length, internode roughness, and internode wall thickness. The relative center of gravity height was calculated by excluding the underground part of the stem, measuring the distance from the base of the stem to its balance point, which represents the height of the center of gravity.

The formulas are as follows:

$$Lodging index(\mathrm{c}\mathrm{m} \mathrm{g}/\mathrm{g}) = Bending moment(\mathrm{c}\mathrm{m} \mathrm{g})/Flexural strength\left(\mathrm{g}\right)$$

(16)

$$\begin{array}{c}Bending moment (\mathrm{c}\mathrm{m} \mathrm{g})\hfill \\ = Length fromthe base of the second internode to the top of the panicle\left(\mathrm{c}\mathrm{m}\right)\hfill \\ \times Fresh weight\left(\mathrm{g}\right)\hfill \end{array}$$

(17)

$$Sectional modulus\left({\mathrm{m}\mathrm{m}}^3\right)=\pi /32\times ({a_1}^3{ b}_1-{a_2}^3 b_2)/a_1.$$

(18)

The major axis diameter (b₁, mm), major axis internal diameter (b₂, mm), minor axis diameter (a₁, mm), and minor axis internal diameter (a₂, mm) of each internode were measured using digital calipers. Folding resistance was determined as follows: the base of each stalk was cut at one or two nodes from the node, specifically at the base of the second node (including the sheath). The sample was immediately placed on a YYD-1A stalk strength tester, aligning the mid-point of the node to the mid-point of the tester (with a distance of 5 cm between the pivot points). The stalk was then pressed downward, and the instrument displayed the readings of the folding resistance.

2.3.6. Yield and Yield Components

At the maturity stage, three representative 1-m² quadrats were selected in each plot to determine the number of the panicles per square meter. From these quadrats, ten hills with uniform panicle numbers were sampled, dried, and followed by the determination of spikelets per panicle, 1000-grain weight, and filled grain rate. Additionally, three random sampling points (excluding border rows and initial sampling rows) were selected per plot, with a total harvested area of 3 m² per point for yield per unit area assessment. The harvested rice was air-dried, threshed to remove impurities, and after which the rice weight and its moisture content were measured. The rice grain weight was converted to the actual yield per unit area at 14.0% moisture content.

2.4. Statistical Analysis

Statistical analyses were performed using IBM SPSS Statistics 26 software. Analysis of variance (ANOVA) included both one-way and two-way analysis of variance, and Duncan’s method was used for multiple comparisons. Differences between the means were statistically significant at the level of <0.05. Figures were generated using Origin 2021, while data organization and table generation were conducted in Microsoft Excel 2019.

3. Results

3.1. Grain Yield and Yield Components

Straw return to the field, nitrogen fertilizer application rate, and their interaction had significant or extremely significant effects on rice yield and yield components; however, their interaction showed no significant effect on 1000-grain weight. Under straw return conditions in the same year, yield increased with higher fertilizer application, showing the trend N3 > N2 > N1 > N0 (

Table 1. Effects of nitrogen application rate on rice yield and yield components under different straw returning years.

Treatment	Effective Panicles (×104 ha)	Spikelets Per Panicle	Filled Grain Rate (%)	1000-Grain Weight (g)	Grain Yield (t/ha)
S0N0	168.00 ± 0.00 fg	105.67 ± 5.84 f	94.56 ± 0.86 ab	23.63 ± 0.81 ab	4.00 ± 0.66 g
S0N1	267.67 ± 0.00 c	132.00 ± 10.20 de	93.78 ± 0.84 ab	23.89 ± 0.46 ab	8.10 ± 0.78 abc
S0N2	306.00 ± 15.59 b	131.67 ± 9.29 de	94.95 ± 0.93 ab	24.03 ± 0.90 a	8.20 ± 0.17 abc
S0N3	350.00 ± 0.00 a	138.22 ± 1.84 de	95.40 ± 2.00 ab	22.61 ± 1.14 bc	8.73 ± 0.12 a
S1N0	160.00 ± 13.86 g	92.44 ± 11.60 g	89.38 ± 1.07 d	22.06 ± 0.97 c	3.43 ± 0.12 g
S1N1	178.89 ± 13.47 ef	153.11 ± 5.59 bc	94.84 ± 1.98 ab	22.88 ± 0.87 ab	6.30 ± 0.20 f
S1N2	241.11 ± 13.47 d	143.78 ± 3.10 cd	95.99 ± 0.68 a	22.77 ± 0.80 ab	7.40 ± 0.20 cd
S1N3	275.78 ± 14.05 c	130.78 ± 2.80 de	94.72 ± 1.39 ab	22.75 ± 0.17 abc	7.43 ± 0.59 cd
S3N0	168.00 ± 0.00 fg	125.22 ± 11.82 e	90.99 ± 2.59 cd	20.11 ± 0.66 d	3.50 ± 0.30 g
S3N1	189.33 ± 0.00 e	163.00 ± 5.20 ab	93.60 ± 2.11 abc	21.98 ± 0.18 c	6.57 ± 0.21 ef
S3N2	261.33 ± 2.31 c	167.56 ± 12.10 a	94.43 ± 1.58 ab	22.46 ± 0.60 c	7.77 ± 1.00 bc
S3N3	312.00 ± 0.00 b	160.44 ± 3.34 ab	92.77 ± 0.54 bc	21.99 ± 0.18 c	8.67 ± 0.67 ab
S	**	**	*	**	**
N	**	**	**	**	**
S × N	**	*	*	ns	**

S0: no straw; S1: straw return to the field for one year; S3: continuous straw return to the field for three years; N0: nitrogen application rate being 0 kg/ha. N1: nitrogen application rate is 125 kg/ha; N2: nitrogen application rate is 150 kg/ha; N3: nitrogen application rate is 175 kg/ha; S: straw return; N: nitrogen application rate. Different lowercase letters indicate significant differences among different treatments (p < 0.05). An asterisk (*) denotes p < 0.05, double asterisks (**) denote p < 0.01, and “ns” indicates no significant difference.

). At the same nitrogen application rate, the rice yield followed the order S0 > S3 > S1, with no significant difference between S0 and S3. This difference was mainly attributed to reductions in the number of panicles and grains per panicle. The number of panicles increased with higher nitrogen application rates, while straw return significantly reduced the effective panicle count. Compared to S0, S1 decreased the effective panicle number by 21.21–33.17%. In contrast, compared to S1, S3 increased the effective panicle number by 5–13.13% and increased the grain yield by 2.04–16.69%. Under the same nitrogen application gradient, significant differences were observed in grains per panicle among treatments with different durations of straw return. As nitrogen application increased, the number of grains per panicle also increased, but no significant differences were observed between different nitrogen application gradients. The 2022 production figures showed the same trend. The yield in the second year followed the same trend (Figure 1).

3.2. Effects of Different Nitrogen Application Rates on Rice Agronomic Traits Under Various Straw Return Durations

3.2.1. Effects of Different Nitrogen Application Rates on Rice Tillering Dynamics Under Various Straw Return Durations

The number of rice tillers increased with the increase in N application, reaching a maximum at approximately 35 days after treatment (Figure 2). Compared with N0, the number of rice tillers increased under N1, N2, and N3, with significant differences observed among treatments with different N application rates. Compared with S0, the number of tillers in S1 decreased by 13.18–22.81%, with no significant difference between S1 and S3. Compared with S1, the number of tillers in S3 increased by 0.88–25%.

3.2.2. Effects of Different Nitrogen Application Rates on Rice Plant Height Under Different Years of Straw Return Conditions

At the tillering stage, under the condition of the same number of years of straw return, different N application treatments increased plant height by 0.56–6.70%, 21.80–34.74%, and 37.02–45.17, respectively, compared with N0, with no significant differences among the different N application treatments (Figure 3). At the nodulation stage, the increases ranged from 10.07% to 40.25%, 11.81% to 40.40%, and 15.80% to 45.37%. At the tasseling stage, the increases ranged from 15.60% to 35.91%, 22.41% to 37.70%, and 17.95% to 36.27%, respectively. Straw return to the field reduced rice plant height with no significant difference among treatments.

3.3. Dry Matter Translocation and Accumulation

Straw return to the field has a highly significant effect on the TVDM of stems and leaves, the TRDM of stems, and the IDM, CRDM, and TDMA of spikes. Nitrogen application rate has a highly significant effect on the TVDM of stems and leaves, TRDM, and the IDM, CRDM, and TDMA of spikes. The interaction between the two has a highly significant effect on the TVDM and TRDM of stems. As shown in

Table 2. Effects of nitrogen application on dry matter transport in rice organs and dry matter accumulation in plants under different years of straw returning.

Treatment	Stem	Stem	Leaf	Leaf	Spike	CRDM (%)	TDMA (kg/ha)
	TVDM (kg/ha)	TRDM (%)	TVDM (kg/ha)	TRDM (%)	IDM (kg/ha)
S0N0	105.60 ± 8.65 d	2.74 ± 0.02 g	198.40 ± 15.43 g	27.24 ± 2.41 b	3149.60 ± 138.00 e	9.64 ± 0.59 e	8319.20 ± 109.49 e
S0N1	965.21 ± 53.44 b	15.53 ± 0.33 cde	846.79 ± 45.52 cd	35.48 ± 3.18 a	6383.36 ± 652.99 bc	28.59 ± 2.05 abc	14,474.89 ± 1240.19 c
S0N2	1215.00 ± 84.87 a	16.60 ± 1.21 bcd	1130.40 ± 110.81 b	39.28 ± 3.03 a	8172.90 ± 366.21 a	28.72 ± 2.88 abc	17,820.00 ± 508.31 ab
S0N3	1237.59 ± 165.83 a	17.65 ± 1.91 b	1364.10 ± 121.32 a	40.10 ± 4.33 a	8745.82 ± 1069.93 a	29.92 ± 2.14 ab	18,589.74 ± 1734.69 a
S1N0	75.20 ± 5.00 d	1.99 ± 0.20 g	157.60 ± 2.77 g	23.40 ± 1.19 b	3587.20 ± 320.67 e	6.52 ± 0.44 e	7840.00 ± 262.30 e
S1N1	608.99 ± 44.46 c	11.98 ± 0.89 f	575.55 ± 25.70 f	35.11 ± 2.10 a	5480.14 ± 771.12 cd	23.19 ± 1.60 d	12,225.71 ± 1056.42 d
S1N2	861.65 ± 62.62 b	15.30 ± 0.21 de	824.33 ± 94.89 de	38.51 ± 4.87 a	6748.59 ± 957.99 b	25.15 ± 2.09 cd	14,162.87 ± 1425.84 c
S1N3	969.96 ± 85.72 b	17.23 ± 1.03 bc	1158.92 ± 96.01 b	39.18 ± 2.24 a	7580.42 ± 1037.19 ab	28.25 ± 2.20 bc	16,147.82 ± 1661.23 bc
S3N0	88.00 ± 9.99 d	2.44 ± 0.17 g	185.60 ± 26.44 g	23.33 ± 2.39 b	2729.60 ± 324.35 e	10.02 ± 0.39 e	7906.40 ± 657.51 e
S3N1	698.18 ± 98.84 c	12.42 ± 0.98 f	695.02 ± 34.49 ef	34.80 ± 3.66 a	4832.01 ± 589.97 d	29.00 ± 3.03 abc	12,277.65 ± 1236.77 d
S3N2	873.60 ± 99.19 b	14.55 ± 1.29 e	968.00 ± 133.76 c	39.70 ± 5.15 a	6511.20 ± 800.51 bc	28.42 ± 2.20 bc	14,538.40 ± 1142.70 c
S3N3	1268.80 ± 175.71 a	19.53 ± 1.86 a	1316.00 ± 85.07 a	40.62 ± 4.75 a	7976.80 ± 204.55 a	32.44 ± 2.89 a	16,803.20 ± 433.07 ab
S	**	**	**	ns	**	**	**
N	**	**	**	**	**	**	**
S × N	**	**	ns	ns	ns	ns	ns

S0: no straw; S1: straw return to the field for one year; S3: continuous straw return to the field for three years; N0: nitrogen application rate is 0 kg/ha; N1: nitrogen application rate is 125 kg/ha; N2: nitrogen application rate is 150 kg/ha; N3: nitrogen application rate being 175 kg/ha; S: straw return; N: nitrogen application rate; TVDM: transport volume of dry matter; TRDM: transport rate of dry matter; IDM: increase in dry matter; CRDM: contribution rate of dry matter transport to the panicle; TDMA: total dry matter accumulation. Different lowercase letters indicate significant differences among different treatments (p < 0.05). Double asterisks (**) denote p < 0.01, and “ns” indicates no significant difference.

, increasing exogenous N fertilizer application significantly enhanced dry matter accumulation and dry matter translocation rate in rice stem sheaths, leaves, and seeds. Under the same nitrogen fertilizer gradient condition, the dry matter translocation rate of rice stem sheath followed the order S3 > S0 > S1, with significant differences among different nitrogen fertilizer treatments. Compared with S0, S1, and S3 decreased the dry matter translocation rate of rice leaves. Dry matter accumulation in the spike increased with the increase in N application, while the contribution of dry matter translocation from the spike to the spike was reduced by straw return to the field in rice. Under different years of straw return, dry matter accumulation followed the order S0 > S3 > S1.

3.4. Effects of Different Nitrogen Application Rates on Nitrogen Accumulation in Various Rice Organs Under Different Years of Straw Return Conditions

Except for the nitrogen accumulation in stems at the full heading stage, straw return to the field, nitrogen application rate, and their interaction had highly significant effects on nitrogen accumulation in various rice organs and the total plant nitrogen accumulation at both the full heading stage and the maturity stage. According to the data in

Table 3. Effects of nitrogen application rate on nitrogen concentration in various organs of rice under different straw returning years.

Treatment	Leaf		Stem		Panicle		Plant
	HS	MS	HS	MS	HS	MS	HS	MS
S0N0	10.43 ± 0.29 j	3.61 ± 0.03 h	10.70 ± 1.37 g	6.85 ± 0.91 g	4.80 ± 0.41 g	25.73 ± 2.04 f	25.93 ± 1.04 i	36.20 ± 2.92 f
S0N1	34.06 ± 1.03 f	12.47 ± 0.21 d	18.90 ± 1.55 e	9.60 ± 1.03 ef	6.70 ± 0.37 f	52.55 ± 5.11 de	59.66 ± 0.62 f	74.61 ± 6.08 de
S0N2	45.40 ± 1.49 c	15.84 ± 0.78 b	22.95 ± 0.00 cd	10.37 ± 0.65 def	9.29 ± 1.22 cd	69.69 ± 0.09 b	77.64 ± 2.68 c	95.89 ± 1.45 b
S0N3	54.09 ± 1.23 a	18.07 ± 0.59 a	31.38 ± 2.35 a	13.60 ± 0.39 b	11.30 ± 0.46 a	82.93 ± 3.39 a	96.77 ± 3.56 a	114.61 ± 4.42 a
S1N0	11.42 ± 0.32 ij	4.20 ± 0.31 fg	14.38 ± 0.92 f	10.99 ± 0.71 cde	7.04 ± 0.32 f	27.13 ± 1.84 f	32.84 ± 0.95 h	42.31 ± 2.83 f
S1N1	23.22 ± 1.23 h	8.34 ± 0.12 e	21.97 ± 0.59 cd	11.97 ± 1.24 bcd	8.68 ± 0.27 de	53.33 ± 3.18 de	53.86 ± 1.41 g	73.64 ± 4.46 de
S1N2	36.16 ± 0.88 e	12.43 ± 0.41 d	22.13 ± 1.36 cd	11.01 ± 0.19 cde	9.62 ± 0.67 cd	57.29 ± 4.71 d	67.91 ± 0.43 e	80.73 ± 5.27 cd
S1N3	49.28 ± 0.23 b	16.22 ± 0.54 b	26.58 ± 1.58 b	12.84 ± 0.68 b	10.12 ± 0.50 bc	63.39 ± 0.20 c	85.98 ± 1.14 b	92.75 ± 1.39 b
S3N0	12.47 ± 0.37 i	4.53 ± 0.26 f	12.98 ± 0.46 f	9.25 ± 0.19 f	7.17 ± 0.70 f	28.65 ± 0.77 f	32.62 ± 0.65 h	42.44 ± 1.21 f
S3N1	31.94 ± 1.03 g	11.77 ± 0.71 d	20.67 ± 1.44 de	12.05 ± 0.79 bc	7.77 ± 0.84 ef	48.98 ± 1.56 e	60.38 ± 1.49 f	72.80 ± 3.04 e
S3N2	38.76 ± 2.26 d	14.18 ± 0.13 c	23.32 ± 1.48 c	12.35 ± 0.43 bc	11.06 ± 0.30 ab	58.55 ± 2.74 cd	73.14 ± 1.22 d	85.08 ± 3.30 c
S3N3	52.47 ± 0.74 a	18.37 ± 0.33 a	31.40 ± 1.22 a	15.48 ± 1.93 a	11.86 ± 0.04 a	81.19 ± 6.69 a	95.72 ± 1.21 a	115.04 ± 8.70 a
S	**	**	ns	**	**	**	**	**
N	**	**	**	**	**	**	**	**
S × N	**	**	**	**	**	**	**	**

S0: no straw; S1: straw return to the field for one year; S3: continuous straw return to the field for three years; N0: nitrogen application rate is 0 kg/ha; N1: nitrogen application rate is 125 kg/ha; N2: nitrogen application rate is 150 kg/ha; N3: nitrogen application rate is 175 kg/ha; S: straw return; N: nitrogen application rate; HS: heading Stage; MS: maturity stage. Different lowercase letters indicate significant differences among different treatments (p < 0.05). Double asterisks (**) denote p < 0.01, and “ns” indicates no significant difference.

, it is evident that nitrogen accumulation in all rice organs increased with increasing nitrogen application, reaching a maximum under the N3 treatment. Nitrogen accumulation in rice plant organs was higher under the S0 and S3 treatments at the flush stage under straw return conditions. At maturity, the treatments showed S3 > S0 > S1, with significant differences between S1, S0, and S3. Compared with S1, leaf nitrogen accumulation under S0 and S3 increased by 11.41% to 49.52% and 13.26% to 41.13%, seed nitrogen accumulation increased by 2.20% to 28.08% and 21.64% to 30.83% and plant nitrogen accumulation increased by 5.39% to 24.03% and 1.32% to 23.57%, and total plant nitrogen accumulation increased by 5.39% to 24.03%, and 1.32% to 23.57%, respectively.

3.5. Effects of Different Nitrogen Application Rates on Nitrogen Transport in Rice Under Different Years of Straw Return Conditions

Nitrogen application rate had an extremely significant effect on NTA, NTE, NTCR, and N increment in the panicle. Straw returning to the field significantly influenced the NTA of stems and leaves, the NTE of stems, and the N increment in the panicles. The interaction between the two had an extremely significant effect on the NTA and NTCR of leaves, as well as the N increment in the panicles. There were significant differences in nitrogen transport in rice among treatments under different nitrogen application conditions. In all treatments, both the amount and rate of nitrogen transport in leaf parts were higher than that in stem sheath parts, and the nitrogen content in all parts of rice at maturity followed the pattern spike > leaf > stem sheath parts (

Table 4. Effects of nitrogen application rate on nitrogen transport in rice under different straw returning years.

Treatment	NTA (kg/ha)		NTE (%)		NTCR (%)		N increment in Panicle (kg/ha)
	Leaf	Stem	Leaf	Stem	Leaf	Stem
S0N0	6.81 ± 0.30 g	3.85 ± 0.48 h	65.32 ± 1.10 abc	36.00 ± 1.27 f	26.61 ± 2.67 c	14.93 ± 0.69 cd	20.93 ± 1.67 f
S0N1	21.59 ± 0.82 e	9.31 ± 1.34 fg	63.38 ± 0.49 c	49.17 ± 5.06 cd	41.23 ± 2.43 b	17.73 ± 2.18 bc	45.85 ± 4.85 de
S0N2	29.56 ± 0.74 c	12.58 ± 0.65 cd	65.12 ± 0.63 abc	54.83 ± 2.83 ab	42.42 ± 1.01 b	18.06 ± 0.959 b	60.40 ± 1.14 b
S0N3	36.02 ± 0.99 a	17.77 ± 2.02 a	66.59 ± 0.87 a	56.54 ± 2.18 a	43.49 ± 2.42 b	21.39 ± 1.56 a	71.63 ± 3.00 a
S1N0	7.22 ± 0.26 g	3.39 ± 0.41 h	63.27 ± 2.14 c	23.56 ± 2.08 f	26.70 ± 1.95 c	12.52 ± 1.51 d	20.09 ± 2.15 f
S1N1	14.88 ± 1.35 f	9.99 ± 0.88 fg	64.02 ± 2.34 bc	45.54 ± 4.62 de	28.02 ± 3.64 d	18.84 ± 2.68 ab	44.66 ± 2.91 de
S1N2	23.73 ± 0.52 d	11.12 ± 1.27 ef	65.62 ± 0.48 abc	50.15 ± 2.60 bcd	41.55 ± 2.62 b	19.42 ± 1.42 ab	47.67 ± 4.12 d
S1N3	32.75 ± 0.31 b	13.74 ± 1.18 c	66.47 ± 0.94 ab	51.66 ± 2.08 abc	51.67 ± 2.35 a	21.67 ± 1.81 a	53.27 ± 0.31 c
S3N0	7.93 ± 0.20 g	3.72 ± 0.31 h	63.64 ± 1.27 c	28.67 ± 1.44 f	27.69 ± 0.84 c	12.99 ± 0.82 d	21.48 ± 0.24 f
S3N1	20.18 ± 0.43 e	8.61 ± 0.76 g	63.19 ± 1.17 c	41.65 ± 1.49 e	41.22 ± 1.23 b	17.58 ± 1.27 bc	41.22 ± 0.76 e
S3N2	24.58 ± 2.22 d	10.97 ± 1.05 ef	63.33 ± 2.03 c	46.97 ± 1.50 cd	42.02 ± 4.06 b	18.70 ± 0.94 ab	47.48 ± 2.46 d
S3N3	34.10 ± 0.65 b	15.92 ± 0.74 b	64.99 ± 0.59 abc	50.81 ± 4.24 bcd	42.20 ± 3.62 a	19.75 ± 2.62 ab	69.34 ± 0.96 a
S	**	**	ns	**	ns	ns	**
N	**	**	**	**	**	**	**
S × N	**	*	ns	ns	**	ns	**

S0: no straw; S1: straw return to the field for one year; S3: continuous straw return to the field for three years; N0: nitrogen application rate is 0 kg/ha; N1: nitrogen application rate is 125 kg/ha; N2: nitrogen application rate is 150 kg/ha; N3: nitrogen application rate is 175 kg/ha; S: straw return; N: nitrogen application rate; NTA: nitrogen translocation amount; NTE: nitrogen translocation efficiency; NTCR: nitrogen contribution rate of translocation. Different lowercase letters indicate significant differences among different treatments (p < 0.05). An asterisk (*) denotes p < 0.05, double asterisks (**) denote p < 0.01, and “ns” indicates no significant difference.

). Under the straw return condition, leaf nitrogen translocation was 10.00% to 45.09% higher in S0 than S1, and 5.63% to 20.26% higher in S0 than S3, with significant differences among treatments. N in spikes increased by 2.67% to 34.47% and 3.30% to 27.21%, respectively. Nitrogen translocation in the leaf and stem sheath parts followed the order N3 > N2> N1 > N0 under the same years of straw return condition. The amount of nitrogen transported followed the order N3 > N2 > N1. These results indicate that both the amount and efficiency of nitrogen transport were higher in the S0 treatment, while the S3 treatment maintained high nitrogen accumulation in rice spikes, suggesting that the nitrogen released from long-term straw return favors nitrogen accumulation.

3.6. Effects of Different Nitrogen Application Rates on Nitrogen Efficiency of Rice Under Different Years of Straw Return Conditions

Straw return to the field has a highly significant effect on NHI, NPP, NAE, NRE, and NAUE. Nitrogen application rate significantly affects NBPE and NPP, and a has highly significant effect on NAE, NRE, and NAUE. The interaction between the two has a highly significant effect on NHI, NRE, and NAUE, and a significant effect on NAE. The data in

Table 5. Effect of nitrogen application rate on nitrogen efficiency of rice under different years of straw returning.

Treatment	NHI (%)	NBPE (kg/kg)	NPP (kg/kg)	NAE (kg/kg)	NRE (%)	NAUE (%)	NPE (kg/kg)
S0N0	——	——	——	——	——	——	——
S0N1	73.39 ± 0.50 a	123.69 ± 17.06 a	60.86 ± 4.50 a	31.20 ± 3.78 b	30.73 ± 2.53 c	30.73 ± 2.53 c	101.44 ± 8.54 a
S0N2	72.68 ± 1.23 a	120.09 ± 9.88 a	61.21 ± 4.21 a	34.46 ± 3.42 ab	39.80 ± 1.03 b	39.80 ± 1.03 b	86.75 ± 10.68 ab
S0N3	72.36 ± 0.85 ab	114.05 ± 15.20 a	61.42 ± 4.10 a	36.74 ± 3.48 a	44.81 ± 0.91 a	44.81 ± 0.91 a	81.91 ± 6.12 b
S1N0	——	——	——	——	——	——	——
S1N1	72.42 ± 1.08 ab	118.68 ± 8.94 a	47.50 ± 3.45 c	24.35 ± 2.91 c	17.90 ± 1.21 f	25.06 ± 1.69 de	97.43 ± 13.29 ab
S1N2	70.91 ± 1.01 b	117.42 ± 8.09 a	50.52 ± 3.42 bc	31.23 ± 2.96 b	25.61 ± 1.66 de	25.61 ± 1.66 de	96.93 ± 7.66 ab
S1N3	68.35 ± 0.48 d	111.66 ± 13.02 a	51.80 ± 2.31 bc	31.46 ± 1.81 b	28.82 ± 0.83 cd	28.82 ± 0.83 cd	96.83 ± 8.19 ab
S3N0	——	——	——	——	——	——	——
S3N1	69.24 ± 0.89 c	119.46 ± 16.33 a	50.84 ± 3.16 bc	20.01 ± 0.74 c	24.29 ± 1.48 e	24.29 ± 1.48 e	97.67 ± 7.78 ab
S3N2	68.80 ± 0.54 c	118.19 ± 25.23 a	56.71 ± 1.99 ab	32.90 ± 1.49 ab	28.43 ± 1.41 cd	28.43 ± 1.41 cd	91.06 ± 10.40 ab
S3N3	67.24 ± 0.82 d	117.59 ± 15.01 a	58.36 ± 1.33 a	36.33 ± 1.90 ab	41.49 ± 4.42 ab	41.49 ± 4.42 ab	88.38 ± 11.95 ab
S	**	ns	**	**	**	**	ns
N	ns	*	*	**	**	**	ns
S × N	**	ns	ns	*	**	**	ns

S0: no straw; S1: straw return to the field for one year; S3: continuous straw return to the field for three years; N0: nitrogen application rate is 0 kg/ha; N1: nitrogen application rate is 125 kg/ha; N2: nitrogen application rate is 150 kg/ha; N3: nitrogen application rate is 175 kg/ha; S: straw return; N: nitrogen application rate; NHI: nitrogen harvest index; NBPE: nitrogen biomass production efficiency; NPP: nitrogen partial productivity, NAE: nitrogen agronomic efficiency, NRE: nitrogen recovery efficiency; NAUE: nitrogen apparent utilization efficiency, NPE: nitrogen physiological efficiency. Different lowercase letters indicate significant differences among different treatments (p < 0.05). An asterisk (*) denotes p < 0.05, double asterisks (**) denote p < 0.01, and “ns” indicates no significant difference. "——" indicates that there is no data for the relevant experimental treatment.

indicates that under the condition of straw return to the field in the same year, the nitrogen harvest index of different nitrogen fertilizer treatments had significant differences and decreased with the increase of nitrogen application. Nitrogen physiological efficiency and nitrogen dry matter production efficiency followed the pattern N1 > N2 > N3, while nitrogen agronomic utilization, nitrogen bio productivity, and nitrogen apparent utilization showed opposite trends. Under the condition of different years of straw return, the nitrogen harvest index followed the pattern S0 > S1 > S3. Under the condition of the same nitrogen fertilizer gradient, successive years of straw return can increase the nitrogen source of the paddy field, and the nitrogen from the straw is absorbed by rice plants, promoting their growth and development.

3.7. Effect of Different Years of Straw Return on the Resistance of Rice to Lodging

3.7.1. The Effect of Different Years of Straw Return on the Rice Lodging Index

From the data in Figure 4, it is evident that under the straw return condition, the lodging index increased with increasing straw return duration. The size of the lodging index followed the order S4 > S3 > S2 > S1, with a significant difference between S1 and S4. Although differences were also present between S1 and S2 as well as S1 and S3 they were not statistically significant. Compared with S0, the lodging index of S1, S2, S3 and S4 decreased by 33.26%, 20.97%, 15.11%, and 2.18%, respectively.

3.7.2. Effects of Different Years of Straw Return on Morphological Traits of Rice Stem

From the data in

Table 6. Main stem morphology and basal internode stem characteristics under different years of straw returning to the field.

Treatment	Plant Height (cm)	Centre of Gravity (cm)	Internode Length (cm)			Internode Diameter (mm)	Internode Wall Thickness (mm)
			I1	I2	I3	I2	I2
S0	118.52 ± 3.61 a	54.42 ± 2.36 a	3.66 ± 1.33 a	10.22 ± 0.92 a	19.20 ± 1.86 a	6.45 ± 0.42 b	0.81 ± 0.053 b
S1	109.56 ± 2.40 c	50.00 ± 2.41 b	2.16 ± 0.30 a	6.40 ± 1.67 b	13.58 ± 1.52 b	7.35 ± 0.85 a	1.01 ± 0.083 a
S2	113.16 ± 3.82 bc	53.08 ± 0.98 a	1.98 ± 0.31 a	8.06 ± 1.32 ab	14.10 ± 1.58 b	6.82 ± 0.72 ab	0.88 ± 0.071 ab
S3	117.48 ± 2.26 ab	54.32 ± 1.14 a	3.16 ± 0.42 a	10.46 ± 0.87 a	18.78 ± 1.67 a	5.94 ± 0.49 b	0.83 ± 0.054 b
S4	118.10 ± 4.38 a	55.10 ± 2.25 a	2.38 ± 0.22 a	9.50 ± 0.88 a	16.38 ± 2.63 ab	6.44 ± 0.39 b	0.81 ± 0.078 b

S0: no straw; S1: straw returns to the field for one year; S2: continuous straw return to the field for two years; S3: continuous straw return to the field for three years; S4: continuous straw return to the field for four years; I1: the first basal internode; I2: the second basal internode; I3: the third basal internode. Different lowercase letters indicate significant differences among different treatments (p < 0.05).

, it is evident that under the straw return condition, the rice plant height increased with increasing straw return duration, with the S1 treatment exhibiting the lowest rice plant height. The rice center of gravity height followed the order S4 > S3 > S2 > S1. The length of internode 2 (I2) at the base of S1, S2, and S4 was reduced by 37.3%, 21.1%, and 7% compared with S0. Under the straw return condition, the internode roughness of S2, S3, and S4 treatments were significantly reduced to varying degrees, whereas S1 significantly increased both internode thickness and internode wall thickness compared to S0. The basal second internode wall thickness decreased gradually with increasing straw return duration. After S0, there was initially an increase followed by a decline. Overall, these results indicate that successive years of straw return reduced rice stem strength.

3.7.3. Effects of Different Years of Straw Return on Mechanical Traits of Rice Stem

Continuous straw fertilization affects the mechanical traits of rice stems. Under the straw return condition, in addition to the breaking resistance of S3, the breaking resistance (a) and sectional modulus (c) of the stalks decreased with increasing straw return duration, while the bending moment (b) and the length from the base of the second internode to the top of the panicle (d) increased with the increase in the number of years of straw return (Figure 5). The breaking resistance of S1, S2, S3, and S4 increased by 52.59%, 48.62%, 5.24%, and 17.29%, respectively, compared with S0. The bending moments of S2, S3, and S4 increased by 4.3%, 17.8%, and 19.0%, respectively, compared with S0. The sectional modulus S1 and S2 increased compared to S0, while S3 and S4 decreased compared to S0. The length from the second basal internode to the panicle apex was reduced to varying degrees, with S1, S2, S3, and S4 decreasing by 6.4%, 3.2%, 2.7%, and 0.1%, respectively.

4. Discussion

4.1. Effects of Different Nitrogen Application Rates on Rice Yield and Yield Components Under Different Years of Straw Return Conditions

The combined effect of straw return to the field and nitrogen fertilizer application delays the senescence of rice leaves, slows down the decline in photosynthetic rate of the midrib leaves during the late reproductive stage of rice, enhances the source–sink relationship, and promotes the transport and accumulation of dry matter [31]. In this study, increasing nitrogen fertilizer application enhanced both the rate and the amount of dry matter transit in stems and leaves (Table 2). With the increase in the number of years of straw return to the field, soil nutrients increased, promoting stalk growth, and increasing dry matter translocation and accumulation, thereby providing a solid foundation for higher rice yield (Table 1 and Table 2). The effect of nitrogen on rice yield is primarily reflected in the effective number of spikes and the number of grains per spike. Previous studies indicate that the number of spikes, the number of grains per spike, the weight of 1000 grains, and the rate of fruiting increase with the increase in the application of nitrogen fertilizer. However, excessive nitrogen fertilizer application reduces the yield. Therefore, a balanced distribution of nitrogen fertilizers is necessary to optimize rice yield [32].

Straw return to the field is an important method of straw utilization, which reduces both the waste of environmental resources and also environmental pollution, while synergistically enhancing soil fertility and yield [33]. At the heading stage and 20 days after full heading, the continuous supply of slow-release fertilizers combined with effective straw decomposition increased both the fruiting rate and the number of effective spikes [34]. In this study, rice yield was higher under the high N application treatment than under the low N application treatment, and increased with the increase in N application. One year of straw return to the field was insufficient, leading to a decrease in the effective number of spikes, and the increase in the number of spikes could not compensate for the reduction in yield. However, consecutive years of straw return to the field improved the total number of spikes and overall yield, despite a slight decrease in effective spikes. Across different years of straw return, the N3 treatment consistently produced the highest yield (Table 1).

4.2. Effects of Different Nitrogen Application Rates on Nitrogen Transport and Utilization in Rice Under Different Years of Straw Return Conditions

Previous studies have indicated that nitrogen fertilizer transport and irrigation methods significantly affect rice yield and quality, and they also exhibit notable interaction effects on nitrogen accumulation in various reproductive organs, with particularly strong effects on nitrogen transport between stem and leaf sheaths [35]. Straw return to the field provides a nitrogen source for rice growth and development, and contributes to nitrogen fixation through its decomposition, thereby improving nitrogen fertilizer use efficiency [36]. In this study, the interaction between nitrogen fertilizer and straw return caused variation in nitrogen accumulation between time periods. Nitrogen accumulation was lower in one year of straw return treatment, likely because straw decomposition consumed available soil nitrogen and exerted stress on soil microorganisms. This effect improved with successive years of straw return (Table 3). Rice showed an increased nitrogen transport rate and nitrogen transport from tillering to maturity, promoting aboveground nitrogen accumulation, which peaked at maturity (Table 3). Straw is a kind of green organic fertilizer, enhancing nitrogen use efficiency and rice yield. Proper nitrogen fertilizer transport combined with straw return to the field can further improve nitrogen use efficiency in rice [37]. Short-term straw return combined with low nitrogen treatment reduced the nitrogen utilization efficiency (Table 5), as straw decomposition reduced the effective nitrogen and affected subsequent rice growth and development [38]. However, three consecutive years of straw return under high nitrogen treatment mitigated this effect. Multi-year straw return improved the soil fertility and enhanced the nitrogen uptake and transport (Table 3 and Table 4), which in turn improved the nitrogen utilization efficiency, contributing to high rice yield (Table 5).

4.3. Effect of Different Years of Straw Return on the Resistance of Rice to Buckling

Since the highest yield was obtained under N3 conditions in the first year of the experiment (Table 1), we investigated its lodging resistance in the second year. Morphological characteristics of rice stalks are closely related to rice stunt resistance, with rice plant height, center of gravity height, and internode length significantly affecting the lodging index [39]. Previous studies found that different levels of nitrogen and potassium fertilizers affect rice plant height under straw-returning conditions, thus affecting rice stunt resistance. Plant height, center of gravity height, and basal internode length were positively correlated with the lodging index, while decreasing the basal internode length and increasing the basal internode wall thickness can improve rice’s resistance to lodging [40]. Under straw-returning conditions, different levels of nitrogen and potassium fertilizers affect rice plant height, and thus rice resistance to fall. This study found that rice plant height increased with higher nitrogen application (Figure 3), and one year of straw return increased rice lodging and decreased the rice lodging index (Figure 4). The S1 treatment reduced rice plant height, center of gravity height, and basal internode length, while it increased basal second internode wall thickness and internode thickness (Table 6). Therefore, straw return to the field can reduce the rice lodging index and improve the resistance of rice to lodge by regulating the factors of stalk morphological traits, including plant height, center of gravity height, basal internode length, and internode wall thickness [41].

Previous studies have indicated that the basal internode resistance is closely related to the distance from the base of the node to the spike apex, bending moment, and folding resistance. Higher stalks, lower folding resistance, and larger bending moments result in a larger inversion index and a lower downing resistance [39]. Continuous straw return to the field over multiple years increases rice plant height and aboveground fresh weight, reduces rice stalk resistance, basal internode roughness, and internode wall thickness, leading to a “head-heavy foot light” condition that increases the risk of rice lodge (Figure 5). The results of this study demonstrate that three consecutive years of straw return (S3) combined with a relatively high nitrogen application rate (175 kg/ha) can increase rice yield and lodging resistance. Therefore, depending on the actual situation of different rice-growing regions, the coordinated effects of nitrogen fertilizer transport and straw return to the field should be applied to enhance rice lodge resistance and achieve high yield.

5. Conclusions

With increasing years of straw return to the field and the amount of nitrogen applied, the number of grains per rice ear increased significantly, resulting in higher rice yields. Long-term straw return increased the dry matter accumulation in all rice parts. Successive years of straw return reduced the number of rice tillers, whereas higher nitrogen application increased the number of tillers and plant height; conversely, plant height decreased as the number of years of straw return increased. Dry matter translocation from leaves and stem sheaths to spikes and nitrogen utilization efficiency increased with increasing nitrogen application. After four consecutive years of straw return, the yield increase was accompanied by an increase in the rice stem lodging index and a decrease in the resistance to lodging, mainly due to a significant increase in the length of the basal second internode and a significant decrease in the internode wall thickness. Therefore, high rice yields and green and sustainable agricultural development can be achieved by reasonably increasing the number of years of straw return and appropriately reducing the amount of nitrogen fertilizer.